ILLUMINATION INDUSTRY 4.0 >

Intelligent lighting for the 21st century | Part 3

DEFECTS IN AUTOMOTIVE PANELS >

From aesthetics to performance,

we can help to ensure quality.

Darkfield backlight illumination

Dark field illumination for detection of defects such as scratches, on
transparent objects, or to check the quality of embossed or raised lettering.

This technique uses a dark field light behind a transparent object to reveal
surface features. It can be used on transparent targets such as glass or
plastic that need to be checked for defects such as scratches, or to check the
quality of embossed or raised lettering. By using this technique, any surface
details are revealed as bright artefacts against a dark background.

This type of backlighting also makes it possible to selectively examine either
the upper or lower surface of a transparent object.

The images produced with dark field backlighting benefit from a very high
contrast which simplifies any subsequent image processing. The example shows a
transparent bowl with heavy scratching on the inside surface.

With a standard backlight, all features are poorly imaged

With a dark field backlight the features are clearly revealed

This image shows how the scratches on the upper surface of the object have
been successfully eliminated to reveal the embossed text on the lower surface
very clearly. To make the task easier, a lens with a small depth of field was
used. Software could now be used to remove the scratches using filter
operations, but this would be CPU intensive and involve more processing time.
Using this technique however allows features on either the upper or lower
surfaces of a clear object to be imaged separately.

Ray diagram of a dark field backlight illumination: It can be used on transparent targets such as glass or plastic that need to be checked for defects such as scratches, or
to check the quality of embossed or raised lettering.